Conventional fluidized bed polymerization reactors may be prone to the formation of “sheets” on the walls of the reactor vessel. Sheeting refers to the adherence of fused resin and resin particles to the walls or dome of the reactor. A correlation exists between reactor sheeting and the presence of excess static charges in the reactor during polymerization. This can be evidenced by sudden changes in static levels followed by deviations in temperature at the reactor wall. When the static charge levels on the catalyst and resin particles exceed a certain level, electrostatic forces drive the particles to the grounded metal walls of the reactor. The residency of these particles on the reactor wall facilitates melting due to elevated temperatures and particle fusion, leading to disruption in fluidization patterns.
The presence of a polymer coating on the bed wall of a gas phase (fluidized bed) polymerization reactor can reduce the tendency of the reactor to form sheets. Without being bound by theory, it is believe that the presence of the reactor wall coating inhibits triboelectric charge transfer that otherwise occurs as the resin in the fluidized bed rubs against the metal reactor walls, which thus minimizes (or reduces) the accumulation of electrostatic charge on the resin. Such a polymer film coating functions as an insulating layer that reduces static charging in the reactor system, thereby reducing the potential for sheeting, during normal polymerization reactions. It is believed that static charging of polymer (e.g., polyethylene) resin in the bed during polymerization is strongly influenced by the electrical interaction between the polymer wall film and the reactor/cycle gas, and is thus strongly influenced by the electrical characteristics of the polymer wall film. For example, a thick insulating wall film would limit charge transfer from the polymer in the bed to ground.
When the polymer coating on the bed wall is in “good” condition, as indicated by its charge decay characteristics (e.g., breakdown voltage), a fluidized bed can be operated for extended periods of time (months or years) without excessive static and without operational problems due to sheeting. However, when the polymer wall coating is in “poor” condition, a considerable amount of static activity can develop in the fluid bed, which often leads to sheeting.
The polymer coatings can deteriorate or become contaminated over time. For example, they can deteriorate as a result of erosion and/or deposition of impurities thereon (e.g., decomposition products of aluminum alkyls). Such deterioration and/or contamination can have a major effect on operability of the reactor. It is conventional to perform reactor system retreatment to remove a deteriorated or contaminated bed wall coating and replace it with a new polymer coating when necessary. Conventional retreatment methods involve preparation of the bed wall (typically by removal of an existing bad polymer coating) and the in situ creation of a new polymer coating on the wall. For example, conventional retreatment methods such as chromocene treatments or hydroblasting may be used. Wall retreatment is expensive and requires shutting down the reactor for the retreatment. Thus, it would be desirable to have a reliable method for monitoring the state of an existing bed wall coating during polymerization operation of a reactor, e.g., to determine when retreatment is or is not needed.
In the past, polymer coatings on the bed walls of fluidized bed polymerization reactors were typically inspected on an opportunistic basis (when the reactors were shut down) by persons who physically entered the reactor vessels with appropriate inspection instruments. Alternatively, the conventional metal coupon approach was used to inspect the coatings but this technique had to be performed in an off-line fashion under conditions not necessarily representative of actual operating conditions. Therefore, there is a need for a method for monitoring (e.g., inspecting and/or characterizing) polymer coatings on bed walls of fluid bed polymerization reactors (e.g., to assess whether the coatings have deteriorated or become contaminated) during performance of polymerization reactions in the reactors (e.g., in on-line fashion during each reaction using a probe external to the reactor). Additionally, there is a need for a method to restore the quality of the wall film without shutting the reactor down for an expensive and time consuming chromocene or hydroblasting treatment.